1. Field of the Invention
The present invention relates generally to a wireless communication system and, more particularly, to a method and apparatus for determining a transmission band for initial access in an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system using a multi-carrier.
2. Description of the Related Art
Normally, an OFDM transmission scheme enables data transmission using a multi-carrier. This scheme is one of the available multi carrier modulation schemes that parallelize a row of symbols that are input in series, and then modulates the parallel rows of symbols to a number of multi-carriers, i.e., sub-carrier channels. The OFDM scheme is similar to a traditional Frequency Division Multiplexing (FDM) scheme, but is inherently characterized by its orthogonality. Namely, OFDM scheme maintains orthogonality among a plurality of tones and therefore realizes optimal transmission efficiency in high rate data transmission.
Additionally, OFDM scheme provides efficient frequency usage and is resistant to multi-path fading, providing optimal transmission efficiency in high rate data transmission.
In wireless communication, data rate and quality of service are dependent upon channel environment. Normally, channel environment in wireless communication may vary frequently due to Additive White Gaussian Noise (AWGN), variations in received signal power caused by fading, shadowing, Doppler effect caused by movement, and changes in velocity of a mobile device, interference caused by other users, multi-path signals, etc. Consequently, it is important overcome such obstructions in the channel environment in order to support a high data rate and high quality data service.
In an OFDM scheme, a modulated signal is located on two-dimensional resources including time and frequency. Resources on the time axis are distinguished by different OFDM symbols, which are orthogonal to each other. Resources on the frequency axis are distinguished by different tones, which are orthogonal to each other. Specifically, in an OFDM scheme, one unit resource is defined by both a specific OFDM symbol appointed on the time axis and a specific tone appointed on the frequency axis. This is often referred to as a Resource Element (RE).
Different resource elements still have orthogonal properties even though they pass through a frequency selective channel. Accordingly, signals can be transmitted to a receiver through different resource elements without causing interference.
A physical channel is a channel on a physical layer for transmitting symbols that are modulated from a row of one or more coded bits. In the OFDMA system, a plurality of physical channels are configured and transmitted according to the uses of transmission data rows or according to receivers. A transmitter and a receiver agree in advance to a rule of correspondence established between a physical channel and a resource element. This rule is often referred to as a mapping rule.
A Long Term Evolution (LTE) system is a representative system in which OFDMA system is applied to a downlink while Single Carrier Frequency Division Multiple Access (SC-FDMA) is applied to an uplink. The LTE system includes a downlink band and a corresponding uplink band.
FIG. 1 illustrates a downlink band and an uplink band in a conventional LTE system.
Referring to FIG. 1, reference number 101 indicates a conceptive structure of an LTE downlink. Reference numbers 111 and 109 indicate transmission of synchronization signals, and reference number 105 indicates transmission of broadcast channel information.
The LTE system defines bands used for a downlink as indicated by reference numbers 131, 135, 137, and 139, and defines bands used for an uplink as indicated by reference numbers 133, 141, 143, and 145. Therefore, LTE system includes a combination of a downlink and an uplink, such as reference numbers 139 and 141.
An LTE Advanced (LTE-A) system is an expanded system of LTE and also realizes a higher transmission rate by continuously using a plurality of bands. The LTE-A system uses a plurality of bands as a downlink and an uplink, respectively. Each individual band may exist in the same band to form continuous frequency bands or may exist in different bands to form discontinuous frequency bands as indicated by reference numbers 121, 123, and 125.
After being powered on, a mobile device determines whether there is an accessible system. For example, a mobile device performs frequency raster scanning to find a specific system connectible with a given frequency. The frequency raster scanning determines whether a synchronization signal exists in a specific frequency by every 100 KHz. Because a synchronization signal has a bandwidth of 1.04 MHz, an LTE system scans 1.04 MHz bandwidth by every central frequency of 100 KHz as indicated by a reference number 103.
When receiving a synchronization signal of 111 and 109 during the scanning, the mobile device obtains system information about the band and then performs initial access. Although a synchronization signal is transmitted, a mobile device sometimes fails to receive the signal and then tries another reception at the next raster.
FIG. 2 illustrates a physical channel structure of a downlink band 211 and an uplink band 212 in a conventional LTE system.
Referring to FIG. 2, a single band includes ten subframes 200 to 209, and a downlink band uses a front part 219 of a subframe for control channel transmission and the other part for data channel transmission. There are two kinds of synchronization signals, namely, a Primary Synchronization Signal (PSS), which is also referred to as the first synchronization signal, and a Secondary Synchronization Signal (SSS), which is also referred to as the second synchronization signal. A PSS is transmitted through subframes 0 and 5, as indicated by a reference number 215, and an SSS is sent in front of the PSS, as indicated by a reference number 213.
Broadcast channel information may be transmitted in two ways; through a broadcast channel, and through a data channel.
In the LTE system, a broadcast channel is referred to as a primary broadcast channel or Physical Broadcast CHannel (PBCH) and transmitted at the rear of the PSS through a subframe 0, as indicated by a reference number 217. A mobile device transmits a minimum amount of information for access to a downlink band through the PBCH. This information is referred to as a Master Information Block (MIB).
Table 1 below shows information transmitted through an MIB.
In Table 1, di-Bandwidth refers to bandwidth information about a downlink, phich-Cong refers to configuration information about a Physical Hybrid Automatic Repeat Request (HARQ) Indicator CHannel (PHICH), and systemFrameNumber refers to a frame number of a received signal.
TABLE 1Information Field in Master Information BlockMasterInformationBlock = SEQUENCE{dl-Bandwidth,phich-Cong,systemFrameNumber,spare}
System information transmitted through a data channel, rather than a broadcast channel, is referred to as a System Information Block (SIB). Normally, there are several SIBs. System Information (SI) required for a system is sorted by its need and transmitted through a plurality of SIBs.
For receiving an SIB, a mobile device receives a control channel region of a subframe by using an MIB. Also, the mobile device finds a transmission location of system information in a data channel by using scheduling information from among system information in a control channel region.
For initial access, the mobile device should receive at least SIB-1 (i.e., the first system information block) and SIB-2 (i.e., the second system information block).
Tables 2 and 5 below show information about SIB-1 and SIB-2, respectively. The remaining information is transmitted through a system information configuration as shown in Table 3, and system information is formed as system information-r8-les, as shown in Table 4, which indicates which SIB is transmitted.
TABLE 2Information Field in System Information Block Type 1SystemInformationBlockType1 = SEQUENCE{cellAccessRelatedInfo,cellSelectionInfo,freqBandIndicator,schedulingInfoList,{si-Periodicity, sib-MappingInfo},si-WindowLength,systemInfoValueTag}
TABLE 3Information Field in System InformationSystemInformation = CHOICE{SystemInformation-r8-IEs,criticalExtensionsFuture}
TABLE 4Information Field in System Information-r8-IesSystemInformation-r8-Ies = SEQUENCE{sib-TypeAndInfo,sibx}
TABLE 5Information Field in System Information Block Type 2SystemInformationBlockType2 = SEQUENCE{ac-BarringInfo,radioResourceConfigCommon,ue-TimersAndConstants,ul-CarrierFreq,ul-Bandwidthmbsfn-SubframeConfigList}
Information received through SIB-1 shows time and period of SIB-2 transmission. SIB-2 transmits information about an uplink band and information about a Physical Random Access CHannel (PRACH) for initial access. PRACH information is included in radio resource config common information of SIB-2, which is shown in Table 6.
TABLE 6Information Field in Radio Resource Config Common SIBRadioResourceConfigCommonSIB = SEQUENCE{rach-Config,bcch-Config,pcch-Config,prach-Config,pdsch-Config,pusch-Config,pucch-Config,soundingRS-UL-Config,uplinkPowerControl,ul-CyclicPrefixLength,}
PRACH transmission for initial access includes information about rach-Config and prach-Config, which are shown in Tables 7 and 8, respectively.
TABLE 7Information Field in RACH-Config CommonRACH-ConfigCommon = SEQUENCE{numberOfRA-Preambles,preamblesGroupAConfig,messagePowerOffsetGroupB,powerRampingParameters,ra-SupervisionInfo,maxHARQ-Msg3Tx}
TABLE 8Information Field in PRACH-Config SIBPRACH-ConfigSIB = SEQUENCE{rootSequenceIndex,prach-ConfigIndex,highSpeedFlag,zeroCorrelationZoneConfig,prach-FreqOffset}
Particularly, rach-config information in Table 7 includes the number of transmittable signals and other information, and prach-config information in Table 8 indicates parameters for initial access signal generation and a transmission band on the frequency axis. Using Tables 7 and 8, a mobile device obtains PRACH resources for initial access at one uplink band. This defines a transmission region 221 at a specific subframe in an uplink band 213.
FIG. 3 is a flow diagram illustrating a conventional procedure for connecting a mobile device to a cell, after initial access.
Referring to FIG. 3, in step 303, a mobile device 301 (also referred to as User Equipment (UE)) performs a frequency raster scanning for a search of a synchronization signal transmitted from a base station 315 (also referred to as an enhanced Node B (eNB)), after being powered on. In steps 317 and 319, the mobile device 301 receives a PSS and a SSS from the base station 315. In step 305, the mobile device 301 detects a physical cell IDentifier (ID) of a downlink band.
In step 307, the mobile device 301 enables channel estimation to receive a channel transmitted from the base station 315. In step 321, the mobile device receives a PBCH in order to detect an MIB, and in step 309, enables a reception of a PDCCH. Receivable information among PDCCH is a common channel transmitted to all mobile devices, and is encoded through System Information Radio Network Temporary Identifier (SI-RNTI) and transmitted in step 323. Each mobile device receives PDCCH carrying system information by using SI-RNTI stored in advance, and receives SIBs transmitted through a data channel by using received information in step 325.
In step 311, the mobile device 301 detects transmission information about remaining SIBs, after receiving SIB-1, and obtains bandwidth information for uplink transmission and initial access resource information for initial access. In step 313, the mobile device configures a signal to be transmitted by using received PRACH resource information.
In step 327, the mobile device 301 transmits a PRACH message 1 (msg1) through a selected PRACH resource. When receiving PRACH message 1, in step 329, the base station 315 transmits PRACH message 2 (msg2) in response to PRACH message 1. PRACH message 2 includes a temporary identifier offered to the mobile device.
In step 331, the mobile device transmits information used for cell connection through PRACH message 3 (msg3), and if cell connection is possible, in step 333, a cell transmits confirmed information about connection through PRACH message 4 (msg4). In step 335, data transmission is allowed between the mobile device and the base station through downlink and uplink.
As described above, an LTE-A system includes a plurality of downlink bands and a plurality of uplink bands. Accordingly, there are several bands capable of detecting an initial synchronization signal in a single system, and also there are several uplink bands capable of transmitting an initial access signal. In view of LTE, such bands are operated with different cells. Therefore, different bands or carriers will be hereinafter used in the same sense as different cells.
When the mobile device performs a frequency raster scanning, most of the mobile devices detect a synchronization signal at a lowest frequency band among downlink bands, and the rest of the mobile devices detect a synchronization signal at a relatively higher band. In this situation, because most of the mobile devices make cell connections through a single uplink/downlink band, resources are unfavorably concentrated in some bands. Also, such a concentrated band deteriorates the performance of the entire system by lowering a probability of initial access, thereby increasing the number of and time required for initial access of the mobile device.